This invention is suitable for use in electrodialysis ("ED") apparatus, with monopolar or bipolar ion-exchange membranes, especially suitable for producing concentrated acids, bases, and salts from dilute salt streams using a direct current driving force. Such apparatus uses ion-exchange membranes to separate, concentrate or transform ions that are usually present in aqueous solutions. The electrodialysis process is driven by a direct current force.
A number of publications and patents describe the membrane technology and the components used to construct the equipment used in the process. Of these publications, the ones judged most informative are:
U.S. Department of Energy Report on "Membrane Separation Systems--A research needs Assessment"; Chapter 8 on Electrodialysis, April 1990. DOE/ER/30133-HI PA1 "Handbook of Industrial Membrane Technology" Ed. by M. C. Porter. Chapter 8, Noyes Publications, 1990 PA1 "Electrodialysis Water Splitting Technology" by K. N. Mani; J. Membrane Sci., (1991), 58, 117-138 PA1 U.S. Pat. No. 5,240,579; 4,871,431; 4,863,596; 4,786,393; 4,737,260; 4,707,240; 4,569,747; 4,319,978; 4,303,493; 4,226,688, 4,172,779; 4,067,794; 3,993,517; 3,985,636; 3,878,086; and 3,679,059
An electrodialysis stack contains an anode and a cathode electrode at its two ends in order to provide an electrical input. Assembled therebetween is a series of membranes and separators (gaskets) secured together in face to face contact, like in a plate and frame filter press. In order to ensure long term reliability of the unit, the electrode chambers may be isolated hydraulically from the main processing unit through a use of a set of membranes and separate fluid circuits.
In a commercial plant, the main processing unit comprises a large number of unit cells, such as 50-250, for example. Each of these unit cells, comprises ion-exchange membranes and solution compartments. Each solution compartment is contained within a gasket, which may be made of a plastic material, such as polyethylene and which may be about 0.5-5 mm thick. These gaskets separate the membranes and provide an adequate sealing at the edges and at other areas, as may be necessary. The gaskets also provide a support for the adjacent membranes and enable a fluid flow into and out of the solution compartment.
Each of the membranes contains manifold holes cut out in the gaskets and "ports" extending from the manifold holes to the solutions compartments in the gasket. The manifold holes and membranes of an electrodialysis stack are aligned to form passage ways which enables the individual process stream to be delivered into and out of the individual solution compartments via the ports.
The unit cells in an electrodialysis stack may be made in a variety of types and sizes. For example, beginning at the anode end for the cell, a first type of unit cell comprises a cation membrane, a dilution or feed compartment, an anion membrane and a concentrate or product compartment. These components form a unit that is used in the desalination of brine solutions and in the recovery and production of salts. Such a unit is called a "standard electrodialysis concentration or desalting cell".
A second type of unit cell comprises a bipolar membrane, a feed or salt/acid compartment, wherein, a feed, such as a sodium salt or an organic acid, is acidified by the H.sup.+ ions generated by the bipolar membrane, a cation membrane that transports the sodium cation, and a base or product compartment where the sodium cation combines with the OH.sup.- ions generated by the bipolar membrane to form a sodium hydroxide (base). Such a unit cell is called a "two-compartment cation cell".
A third type of unit cell comprises a bipolar membrane, a product or acid compartment, an anion membrane and a feed or salt/base compartment. Such a unit is called a "two-compartment anion cell" and may be used to basify an ammonium salt solution in order to generate an acid product and an ammonia rich base solution.
Yet another type of unit cell comprises a feed or salt compartment, a cation membrane, a base compartment, a bipolar membrane, an acid compartment and an anion membrane. Such a unit is called a "three-compartment bipolar cell" and may be used to convert a salt, such as sodium chloride, into sodium hydroxide and hydrochloric acid.
Unit cells containing more than three membranes and three compartments are also known. Excluding electrode rinse loops, a two-compartment cell can handle two process streams while a three-compartment cell can handle three process streams and so on. When a direct current is passed through the central ("active") area of an electrodialysis stack, the ions contained in a solution migrate in the direction of the current. The cations cross the cation membrane and move toward the cathode or negative electrode, while the anions move across the anion membrane and toward the anode or positive electrode. If a bipolar membrane is deployed in the electrodialysis stack with the cation side facing the cathode, the direct current input accelerates the dissociation of water at the membrane's interface. As a result hydrogen (H.sup.+) and hydroxyl (OH.sup.-) ions are concentrated on the cathode and anode sides, respectively. Depending on the cell configuration and the process deployed, the end result is a concentration of a salt from a dilute stream or a depleted salt stream or is a conversion of a salt into its acid and base components.
While the performances of the specific ion-exchange membranes that are deployed and the adequate pretreatment of the feed stream, (e.g. pH adjustment, filtration etc.) are important to a successful operation of electrodialysis process, an equally important aspect is the flow and distribution of the fluids inside the gasket or separator.
Commercially the two major electrodialysis designs in common use are called "sheet flow" and "tortuous flow".
Examples of sheet flow separators are sold by Asahi Glass, Tokuyama Soda, and by Aqualytics, a division of Graver Water. The active area is usually 50-85% or more of the overall gasket area in order to maximize the membrane utilization. This active area is open to both fluid and electrical flow. The membrane itself is supported by a non-woven "mesh" material having approximately the same overall thickness as the gasket. The mesh also distributes the fluid in the active area. To further ensure a uniform fluid distribution across the length or width of the active area, a multiplicity of feed manifolds and ports are employed. In these conventional stacks, the pressure drop and membrane strength considerations dictate that the superficial linear velocity of the liquid be kept low, typically in the order of 5-10 cm/sec.
Experience indicates that the distribution of fluids over a large cross sectional area can, as in many commercial stacks employing the "sheet flow" design, be a problem when the process is operated at high electrical current densities (&gt;50 mA/cm.sup.2) or at lower stream conductivities (&lt;20 mS/cm). Other problems occur with membrane swelling or wrinkling, particularly when there are even trace amounts of precipitable substances in the feed stream. Further, the conventional use of multiple manifolds and ports aligned in a stack of gaskets increases the intercompartment leakage.
Shunt and stray electrical current problems result in process inefficiencies which can cause an overheating or meltdowns of gaskets. This problem usually limits to about 100 the number of unit cells that can be safely deployed in series, without requiring some kind of electrical current interruption structure located between electrodes in a commercial stack operating with highly conductive solutions or at high current densities.
The overall flow rate requirements are high, due to the need to maintain the requisite linear velocity over large cross-sectional flow area. This need, in turn, requires the use of larger pumps and recirculation tanks. If mechanically weak (e.g. 20-45 psi burst strength), the prior art membranes are poorly supported in the large active area. They are easily damaged or broken over a long term operation due to the pressure variations in the feed streams.
Ionics Inc. uses electrodialysis stacks having a tortuous flow electrodialysis design employing gaskets having a long liquid flow path. Individually, the Ionics flow channels are quite narrow, (typically 1-1.5cm). They make several 180.degree. turns between the entrance and exit ports. The Ionics ports themselves are simply slots formed in the gaskets. The membranes are mechanically stiff structures in order to prevent their collapse into and the resultant blockage of the ports.
To overcome polarization problems associated with the low conductivity feeds, a higher linear flow velocity is used, typically 30-50 cm/sec. There are high pressure drops in such stacks (as high as 3-6 bars vs.0.5-2 bar for the sheet flow stacks). Consequently, the conventional membranes for the tortuous flow stack s have been thick and mechanically sturdy. Typically, they have a high electrical resistance leading to high power consumption.
Tortuous flow stacks are suitable for desalination applications which operate at a low electrical current density (0.1-10 mA/cm.sup.2). For chemical production applications, such as a conversion of sodium chloride to caustic soda and hydrochloric acid in a three compartment cell which operates at high current density (30-200 mA/ cm.sup.2) ,the amount of heat generated and the resulting temperature increases are unacceptable when using tortuous flow stacks unless the active areas of the stack are small. This limit on the active area size, in turn, requires the use of a large number of stacks in a commercial plant, with an attendant high cost. The stacks are also prone to intercompartment leakage because the membranes swell and spall.